The present invention relates to an induction motor power supply control apparatus, and in particular to an induction motor power supply control apparatus providing variable-speed operation of an induction motor, together with a high degree of efficiency and with complete stability of operation over a wide range of motor load.
In the prior art, various proposals have been made for controlling the power supplied to an induction motor which must drive varying levels of load, in general by controlling the operation of a DC-AC power inverter (hereinafter referred to simply as a power inverter) to determine the supply voltage of the motor. Among these are proposals to obtain increased efficiency of operation of an induction motor by controlling the supply voltage such that the motor operates with a fixed value of power factor (typically, approximately 80%), irrespective of changes in the load imposed on the motor. However in practice it is found with such a method that although the operation is satisfactory when the motor is driving a high level of load, the power factor is excessively high when the motor is operating at an intermediate level of load or light load. That is to say, with such a method the value of supply voltage applied to the motor under a condition of intermediate load or light load is made excessively low, which can resulting an an excessive degree of motor slip, an excessive level of motor current, and hence increased power consumption when operating at an intermediate level of load. The objective of achieving increased efficiency is thereby defeated, while in addition the motor speed will become unstable when the motor is operating under light load or zero load, and the motor may stall.
Such problems of the prior art become especially severe in the case of a variable-speed induction motor which must not only function with efficiency and stability over a wide range of load values, but also at a number of different values of supply frequency. An induction motor control apparatus for controlling a variable-speed induction motor is described for example in Japanese Patent Laid-open Publication No. 64-50792 whereby respectively different values of optimum power factor are established for various values of motor supply frequency. The supply voltage of the induction motor is controlled to operate, at any particular value of drive frequency, fixedly at the power factor that has been predetermined for that frequency. However for the reasons described above, such a method cannot provide high efficiency and stability of operation for a motor which must drive a wide range of varying load values.